Separation of iron from aluminum sulphate solutions



Patented Feb. 25, 1947 SEPARATION OF IRON FROM ALUMINUM SIlLPHATE SOLUTIONS Robert B. Barn"es, Stamford,'and Garnet Ham, Old Greenwich, Conn., assignors to American Cyanamid Company, New York, N. Y,, a corporation of Maine N Drawing. Application May 13, 194 1, Serial No. 535,540

7 Claims. (Cl. 23-118) 1 The present invention relates to the separation of iron from Al2(SO4)3 in aqueous solutions.

In the metallic aluminum industry, where the metal is normally produced from alumina, it is important that iron' be eliminated from the alumina so as to leave the former in as pure a state as possible. I Many proposals have been made for the precipitation. of iron at an intermediate stage in. the conversion of bauxite, clays and otheraluminum bearing earths, to alumina. These methods are cumbersome, and inefiicient inasmuch as they involve large'quantities of liquids and usually contemplate precipitation of the iron as the hydroxide, which is. most diflicult to filter, Therea'gents used arebulky, which also gildis to the total volume of material to be han- The principal object of the present invention is toremove iron from A1z( S O4)a solutions in a simple manner without adding greatly to the bulk of materials handled.

- The invention is based upon the discovery that the formation of an aluminum xanthate requires a greater length of time than doesthe' xanthate of iron. As a:consequence, by adding a water soluble ester of xanth'ic acid to an'aqueous solution containing Al2( SO4)3 and iron under proper controlled conditions, ,the iron may be thrown out of solutionbefore. any appreciable amounts of aluminum xanthate' have been formed. Thus, the ironlxanthate may be readily separated from the liquid without aluminum loss. V .l There are a number of important factors which contribute largely to the success of the, herein described method. For instance, it has been found that best results are obtainable whenflthe pH of the solution from which the ironis being precipitated is in the neighborhood of 3..5.j It has also been found that the iron maybe moreefiiciently precipitated when it is in the ferric state and, consequently,,steps should be' taken to convert ferrous iron to ferricjiron prior toprecipitation. Again, during the xanthation process, the iron precipitate normally is'colloidal in nature and thus difficult to filter.v If,however, precipitation takes place with strong agitation, anagglomera; tion of the precipitated iron particles occurs with the result that a crystalline or granular form is assumed by the precipitate. and the latter may idizing agent "such as potassium permanganate,

sodium perchlorate, hydrogen peroxide, manganese dioxide, calcium hypochlorite or theilike to,

convert any ferrous iron to the ferric, state. A water soluble ester of "xanthic acid such as any of the alk-alimetal-alkyl Xanthates and particularly sodium ethylxanthate, is then added with.

agitation. The iron isfirst precipitated as an abundant dark brown, finely divided precipitate in a highly dispersed state. Upon agitatiomhowever, this materialagglomerates to form a heavy black crystalline precipitate, apart of which'set-j tles rapidly..while the remainder floats. ,Bothfl portions of the solids may be readily filtered and under, the above circumstances, only small amounts-"of aluminumare found inthe precipi-t tate while the contenti'oi' iron in the'solution has, beenappreciably reduced I l .j The iron xanthate precipitate upon Qbeing treated with an alkali, such: as ammonium hydroxide-may be reconverted into a'water-soluble xanthate and substantially pure F6203. The latter may be separated byfiltration and the former used as make-up xanthate' in a cyclic process.

Any aqueous solutions of A12(SO4) 3 may be thus treated to remo'veits iron component; However, inasmuch as aqueous solutions of Al2(,SO4) 3 have a low pH, and xanthic acid esters are more or less unstable under these conditions, it is desire able to adjustthe pH oftheAMSOQzsolution to a point where xanthate 'decompositionis minimized. Actually this'point'cannot be reached because if the pI-Ibecomes too high, not only is the iron precipitated as hydroxide but'the 3/1119 minum comes down as the hydrate-as well. However, at a pI-I 'of 3.5, whiletl'ie acidity'is such that some xanthate is lost through decomposition yet this point approaches the optimum conditions between loss of aluminum through hydrate precipitation and loss of xanthate,through acid decomposition) As a consequence, it is always desirable to use an amount of xanthate'more than theoretically necessary to react with theiron present, calculated as F6203. This theoretical ratio of xanthic acid ester to iron is approximately 5.2: 1. The amount of xanthate thus used should be greater than" this ratio to compensate The same ratios hold for multiple operations cal- V g oulated as total xanthate to iron, all as will be 1 illustrated in the examples.

Normally, most aqueous solutions of A12(SO4)3 filtrate when analyzed spectrophotometrically' have a pH below 3. They may be brought to the 1 arbitrary 3.5 point by any type of alkali which 1 will not cause, apreeipitation of the iron or aluminum as hydrate.

I if added as a solid, no undue addition to the bulk,

of materials handled is involved.

Experiments have demonstratediithat the ironcontent of AI2(SO4)3 solutions is inefficientlfiprfia cipitated unless the iron is in the ferric state. As.

Soda ash is preferred and thate ratio of to 1.

, tering thez'clear solution was used for a second a consequence, oxidizing agents-maybe usedi;

although best results have beeneohtained; with a;

i 10% solution of potassium permanganate,'adding the same with stirring' until .a slight pink color persists. I

ester as a, 1.0%-- aqueous solution.

-Any alkali metal-alkyl xanthate may be usedi Desirable results have been obtained notonly;

with sodium ethyl xanthate but also with sodium isopropvlxanthata. potassium amyl xanthate and i the like. Hist-desirable touse the;xarrtln"c"acid 1 When. the xanthic, acid estel'j s d ed to-th'e conditioned Al'ziSOr) a solutiomcontaining ferric iron. an. abundant and.spontaneougprecipitate of. an. iron xanthate immediately formed. 1 While somealuminum xanthate' i's-a-lso formed;

yet this material occurs'iin exceedingly small amounts. andonly after a protracted time period;

1 This-difference inthe precipitatei forming times 1 permits, removal of the iromxanthate by'rapidfiltration,without' loss ofalumin'um;

When the iron. is first precipitated as xanthate;

I it comes. down as adark brownyfinely divided'dispersion which is extremely-difiicult to filter and 1 will not settle out on a' standing"period short of aluminum xanthate formation. It was discov ered, however, that iffeither'the menstruum con- 1 taining the dispersed" iron 'xanth'ate' isviol'ently agitated or if agitation is;continued duringthe j xanthate addition, tlie' dark "brown dispersiorr'i's converted to heavy black c'rystallin'e' agglomer showed P. P. M. iron remaining. This represents a 93% removal.

I EXAMPLE II 50 ml.. of the same solution" asa'above containing 1360 P. P; M. total ironwas'treated with 5 ml. of,,10% sodium ethyl Xanthate. This was a Xan- After agitating and filsttage; o fixanthation.

Thecleansolution was oxidized with 10% potassium; permanganate until a pink color perwas added; I After agitation and filtering, the clear. solutioncontained 59 P. P. M. of total iron. This-represented a 95% removal with a totalxanthate ratio to iron of 12: 1.

EXAMPLE III A portion of the alum. solution oi' Examme I was.- preoxidi'zedjwith ,1Q'%l"bota ssium per.- manganate untilja pink; colorpersistedj 11.0 ml:

oithe solution was then treatediwith 0.4imlif; oi,

10% sodium ethyl .xanthate solution This: rep resenteda ratio o f 2:1.. After agitation, it' was filtered'and the filtrate, used in Stage 2'."

The clear solution was oxidizedandxanthated the second time, followingthe, above procedure;

and the filtrate, used? for a (third-stage;

In this final stage... oxidation and Xanthation. took place as.above,.,giving, a .final solution'which upon. xanthation, showed only, 13 P; P; ,M. of iron. remaining. Thisvrepresents a 9.9%;- rem'ovall. of.

iron, usinga .totali-xanthatearatio.ofj6': LCeXtendedI over three..stag es..

ates, some of which" float andsome-of" which 1 sinkbut-all oiwhich may-bereadily'and quickly 1 of any appreciable amounti-ofraluminumxanthate;

It'is "to be noted ate-this point-thatftlredronchas 1 been removed without "adding-materially: to the bulk'of the originalfsolution+as thevonly liquid-s added are the oxidizing agent: and-the xa-nthic acid ester:

paratively small; v v 3 1 The'iron xantliatencake; upon being-treated 1 with an alkali such' as:ammjonium-hydroxide; is 1 reconverted'to a water solub1e=xanthate,-.leaving i a relativelypure FezOx'b e'hinda ,Th-iS SOlllll'lOl-LOf x'anth'ate'maybeused as SHCh 'IIh'thBEIUHOWIHg 1 cycle or itmaybemixed witlrone'ofrtherabove These: volumes: however; are com:-

mentioned xanthatesr for theesame purpose Q Throughout the?- examples: where the'ewor'd alum 'isused'f AlziSOailaziS meant I earm rk r 500.: m1; 01ft at. crude-s1 alum, solution (-323 B., EH1 2.3 and: from-V which...-thesolids, have, been filterewawas oxi'dizedin thei coldtby4Atmlioiia potasslum,-pez'ar a'nganateT solution; I 1 total: iron; content was% .l36.0+parts per million. 10 ml. of this solution weretreatedwith 3..ml-.-. of 1 aeI z sodium ethyl xanthate solution... This represented-a ratio; of :xanthate;tolironrcalcue latedyasl- FezQs' of; 10;1.. Thewsolutionmwastagili atednvigorouslyyand... then: filtered The. clear The filtered from the soluti'onprior-tmthe formation uteseof operation; the solution, was filtered" and reservedgforthesecond stageof' xanthation; I

In the second istage, the above. solution; was oxidizedas .before' and; I60 m1; offsodiurn ethyl xanthateradded." Thisgigaxratio of 31:1. The aeration and floatation was continued ior twentyrfour minutes the. solution filtered, and

reserved for the third stage ofv xanth'atio'n;

In ,tl'iis'tfinal stage, the solution was reoxidized as before and.il10-ml..of;10'% sodium ethyl xan= thateaddedr This represents a ratidoi 2.11.." No frothing ,agentwasnsedin tl'iisgstageg; The aerationeandfioatatibn operation lasted for. fifteen minutes. Upon filtering,,thfsolution and analyzingiit for iron,.itiwas foundithat 15TH PQM": remained; Thisisea .9'6L% removalycfjroniwith a total; ratio of'xanthate t'o ironicalculatedas FiaOaniHlJ-i. I

I EXAMR'LEZEVE 250ml; ofthealum'sohition used in Exam-p IV, were placed in a laboratory,flotation machine having a pushelt type propeller and "air *cli'ifu'ser and'ijsurfacevanesi f I i to create a" surface" swirling-action. The solutibn was'not preoxidizedf' 126i.grams offso diuni etwl Stage 2 The filtrate from the above was oxidized with permanganate and then 2.0 grams of 10% sodium ethyl xanthate in 5 ml. of water. which represented a xanthate-iron ratio of 5:1. Pine oil was again used as a frothing agent. After fifteen minutes of operation similar to that used for Stage 1, the solution was filtered to free it from the precipitate and reserved for Stage 3.

Stage 3 The above solution Was oxidized with 10% permanganate. 1.2 grams of 10% sodium ethyl xanthate in 5 m1. of water and representing a ratio of 3:1 were used for xanthation. Pine oil was used as a frothing agent. After fifteen minutes of agitation and aeration, the solution was treated with 0.1 m1. of 5% copper sulfate. After a short standing period, the entire solution was filtered to remove all precipitate.

The original solution contained 1150 p. p. m. of iron. After the first xanthation, this had decreased to 810 p. p. m., after the second to 175 p. p. m., and after the third, to 18 p. p. m. This represented a total iron removal of 98% with the xanthate to iron ratio of 12:1.

EXAMPLE VI 40 ml. of the alum solution of Example IV were treated with six grams of solid sodium oarbonate to adjust the pH to 3.5. This solution was then treated in three successive xanthation stages, using sodium ethyl xanthate in the respective ratios of 4:1, 3:1 and 3:1. The initial solution contained 1150 p. p. m. of tota1 iron,

which was decreased after the three stages of xanthation to 435 p. p. m., 145 p. p. m. and 30 p. p. m., respectively. No oxidation was used in this experiment.

EXAMPLE VII Example I was repeated, using sodium isopropyl xanthate in a single xanthation stage in a ratio of 10: 1. After agitation and filtering, the clear solution contained 85 p. p. m. total iron.

EXAMPLE VIII 5 parts by weight of an iron xanthate as obtained in Example VI were placed in a filter funnel and ten parts by volume of 10% ammonium hydroxide were poured in. In a short period of time, the dark brownish-black precipitate gradually changed to an orange-red, and the filtrate obtained was a yellow-green color.

The entire quantity of filtrate was added to 10 ml. of an alum solution containing 1285 p. p. m. of total iron. After agitation and filtration,

the clear solution was found to contain only 63 p. p. In. total iron, which represented a removal. g

Thus, the xanthate may be regenerated and recycled with efficient results.

While the invention has been described with particular reference to specific embodiments, it is to be understood that it is not to be limited thereto, but is to be construed broadly and restricted solely by the scope of, the appended claims.

What is claimed:

1. A method of precipitating iron from a solution containing it and A12(SO4)3 which includes adjusting the pH of the solution to not less than 3.5 and adding thereto, a water soluble alkyl xanthate.

2. A method of precipitating iron from a solution containing it and A1z(SO4)s which includes adjusting the pH of the solution to not less than 3.5 and adding thereto a water soluble alkyl xanthate in a ratio to the iron content greater than 5.2 1.

3. A method of precipitating iron from a solution containing it and A12(SO4)3 which includes adjusting the pH of the solution to not lessthan 3.5, adding an oxidizing agent to convert ferrous iron to the ferric state, and adding thereto a water soluble alkyl xanthate.

4. A method of precipitating iron from a solution containing it and Al2(SO4)3, which includes adding thereto sodium ethyl xanthate.

5. A method of precipitating iron from a solution containing it and A12(SO4)3 which includes adjusting the pH of the solution to not less than 3.5, adding thereto a water soluble alkyl xanthate and agitating the mixture until the iron xanthate has agglomerated.

6. A method of precipitating iron from a solution containing it and Alz(SO4)3 which includes adjusting the pH of the solution to not less than 3.5 and adding thereto, a water soluble alkyl xanthate, with agitation.

7. A method of precipitating iron from a solution containin it and A12(SO4)3 which includes adjusting the pH of the solution to not less than 3.5 and adding thereto, a water soluble alkyl xanthate, with agitation, and removing the iron xanthate before the formation of a substantial amount of aluminum xanthate.

ROBERT B. BARNES. GARNET P. HAM.

REFERENCES CITED The following references are of record in-the file of this patent:

UNITED STATES PATENTS 

